With the fast growth of portable electronics and equipments, the demand for a DC-DC converter that is lower in cost, higher in efficiency and requires smaller board area is also increasing. To use smaller external components, a high switching frequency is often required. However, it is often difficult to achieve higher switching frequency with the conventional constant-frequency current mode control due to the delays of the PWM comparison circuitry. The conventional control also has the drawback of slow load transient response, and the conventional control is difficult to compensate.
A conventional DC-DC converter is shown in FIG. 1. In this circuit, a DC-DC converter IC 100 regulates an output voltage via a resistive feedback divider made up of resistors 108 and 109 between the output voltage and a ground node GND. A center tap of the resistive divider is connected to a feedback amplifier 120 that amplifies the difference between a feedback signal and a reference voltage from a voltage reference 101. The signal at the output of feedback amplifier 120 is then converted into a duty-cycle signal by a controller 121 that bases the switching frequency on an oscillator 122. This duty-cycle signal then drives a main switch 105 and a synchronous rectifier 106, generating a voltage pulse waveform at an inductor switching node LX. An inductor 112 and an output capacitor 113 act as a low pass filter to filter out the voltage pulse waveform to supply to the output. Because of the negative feedback effect, the output voltage is regulated so that the feedback voltage is essentially equal to the reference voltage.
The disadvantage of IC 100 is that to stabilize the feedback loop, the feedback gain must not be too high, especially at a frequency between 1 kilo-Hertz and 100 kilo-Hertz. This results in a slow response to the variation at the output voltage due to a sudden load change or input voltage change. The variation at the output is also first reduced through the resistive divider before going through the amplifier, resulting in a weaker signal for feedback. Furthermore, the delays in feedback amplifier 120 and controller 121 both result in a limitation on how high of a switching frequency the IC can operate.
FIG. 2 shows another example of a prior art DC-DC converter that results in higher frequency operation than the circuit shown in FIG. 1. In this architecture, a feedback resistor 208 is connected between an inductor switching node LX and a feedback node FB to provide a DC feedback path. In addition, a feed-forward capacitor 211 is connected between an output voltage 220 and a feedback voltage to provide a high frequency feedback path. A hysteretic feedback comparator 202 then compares the feedback voltage with a reference voltage from a voltage reference 201 and generates a signal to control a main switch 205 and a synchronous rectifier 206 via a controller 204. The response of this circuit is faster than the circuit of FIG. 1 because it is easier to stabilize, because the output voltage variation is forward-fed into the feedback node FB, and because there is less delay in the loop. In this circuit, the switching frequency is a function of feedback resistor 208, feed-forward capacitor 211, feedback comparator 202 hysteresis voltage, the input voltage 214, and output voltage 220.
Unfortunately, such conventional approaches as shown in FIG. 2 suffer from the problem that the output voltage is sensitive to the resistance of an inductor 212 and the switching, frequency varies with the input and output voltages. Since the DC feedback point is set by the resistive dividers from the inductor switching node LX, the output voltage changes by a value equal to RIND * IOUT, where RIND is the DC resistance of inductor 212, and IOUT is the output current. For instance, assuming a typical inductor resistance of 100 mΩ and an output Current of 1A, the voltage drop or load regulation is 100 mV, which is unacceptable in many applications.
In view of the foregoing, there is a need for a solution to achieve a high switching frequency that is relatively constant and has significantly reduced load regulation by introducing a duty-cycle-controlled hysteretic switching scheme and by using a unique feedback network.
Unless otherwise indicated illustrations in the figures are not necessarily drawn to scale.